The present invention relates generally to a controller for administering a rotary sterilization process being performed on line of containers. In particular, it pertains to such a controller that also provides on-line handling of a deviation in a scheduled parameter during the process by identifying any containers that will be under processed as a result of the deviation.
A rotary sterilization system is a continuous source processing system with intermittent product agitation. This system is widely used in the canning industry to sterilize a shelf stable food product packaged in containers. It is used most often for sterilizing a food product that benefits from mechanical agitation of the containers.
A rotary sterilization system includes a rotary sterilizer that has one or more cooking shells through which a line of containers {1, . . . , i, . . . , I}line are conveyed. The containers are cooked in the cooking shell(s) at one or more scheduled cooking retort temperatures. The containers are then conveyed in line through one or more cooling shells of the rotary sterilizer. Similar to the cooking shell(s), the containers are cooled in the cooling shell(s) at one or more scheduled cooling retort temperatures.
The containers {1, . . . , i, . . . , I}line are conveyed through each cooking and cooling shell by spiral tracks and a reel. The reel has a scheduled reel speed and imparts movement while the spiral tracks provide the direction for the containers to be conveyed through the shell. This also provides mechanical agitation of the food product within the containers.
In order for the food product in each container i to be commercially sterilized, a total lethality Fi over a total time interval [tf,i, td,i] that satisfies a predefined target total lethality Ftarg must be delivered during the rotary sterilization process to the product cold spot of the container. Here, tf,i and td,i are the feed and discharge times when the container is fed into and discharged from the rotary sterilizer. The target total lethality is set by the USDA (U.S. Department of Agriculture), the FDA (Food and Drug Administration), and/or a suitable food processing authority for destroying certain microorganisms. The reel speed and the cooking and cooling retort temperatures are then scheduled so that each container i will receive a time temperature treatment according to a scheduled time-temperature profile that delivers a total lethality to the container which satisfies the target total lethality.
As is well known, the lethality Fi delivered to the product cold spot of a container i over a particular time interval [tm, tk] is given by the lethality equation:       F    i    =            ∫              t        m                    t        k              ⁢                  10                              (                                                                                T                    cs                                    ⁡                                      (                    t                    )                                                  i                            -                              T                REF                                      )                    /          z                    ⁢              ⅆ        t            
where tm and tk are respectively the begin and end times of the time interval [tm, tk], TCS(t)i is the product cold spot timne-temperature profile for the container, z is the thermal characteristic of a particular microorganism to be destroyed in the sterilization process, and TREF is a reference temperature for destroying the organism. Thus, the total lethality Fi delivered to the product cold spot over the total time interval [tf,i, td,i] due to the scheduled cooking and cooling retort temperatures is given by this lethality equation, where tm=tf,i and tk=td,i.
The total time interval [tf,i, td,i] and the product cold spot time-time-temperature profile TCS(t)i must be such that the total lethality Fi over [tf,i, td,i] satisfies the target total lethality Ftarg. In order to ensure that this occurs, various mathematical simulation models have been developed for simulating the product cold spot time-temnperature profile based on the scheduled retort temperatures. These models include those described in Ball, C. O. and Olson, F. C. W., Sterilization in Food Technology: Theory, Practice and Calculations, McGraw-Hill Book Company, Inc., 1957; Hayakawa, K., Experimental Formulas for Accurate Estimation of Transient Temperature of Food and Their Application to thermal Process Evaluation, Food Technology, vol. 24, no. 12, pp. 89 to 99, 1970; Thermobacteriology in Food Processing, Academic Press, New York, 1965; Teixeira, A. A., Innovative Heat Transfer Models: From Research Lab to On-Line Implementation in Food Processing Automation II, ASAE, p. 177-184, 1992; Lanoiselle, J. L., Candau, Y., and Debray E., Predicting Internal Temperatures of Canned Foods During Thermal Processing Using a Linear Recursive Model, J. Food Sci., Vol. 60, No. 4, 1995; Teixeira, A. A., Dixon, J. R., Zahradnik, J. W., and Zinsmeister, G. E., Computer Optimization of Nutrient Retention in Thermal Processing of Conduction Heated Foods, Food Technology, 23:137-142, 1969; Kan-Ichi Hayakawa, Estimating Food Temperatures During Various Processing or Handling Treatments, J. of Food Science, 36:378-385, 1971; Manson, J. E., Zahradnik, J. W., and Stumbo, C. R., Evaluation of Lethality and Nutrient Retentions of Conduction-Heating Foods in Rectangular Containers, Food Technology, 24(11):109-113, 1970; Noronha, J., Hendrickx, M., Van Loeg, A., and Tobback, P., New Semi-empirical Approach to Handle Time-Variable Boundary Conditions During Sterilization of Non-Conductive Heating Foods, J. Food Eng., 24:249-268, 1995; and the NumeriCAL model developed by Dr. John Manson of CALWEST Technologies, licensed to FMC Corporation, and used in FMC Corporation""s LOG-TEC controller.
However, if any of the actual retort temperatures in the cooking and cooling shells drops below a corresponding scheduled cooking or cooling retort temperature, a temperature deviation occurs. Traditionally, when such a deviation occurs, the controller stops the shells"" reels and prevents any of the containers {1, . . . , i, . . . , I}line from being fed into or discharged from the rotary sterilizer until the deviation is cleared. But, this approach causes numerous problems. For example, significant production down time will result. And, many containers { . . . , i, . . . }overpr will be over processed since the total lethalities { . . . , Fi over [tf,i, td,i], . . . }overpr actually delivered to their product cold spots will significantly exceed the target total lethality Ftarg. All of these problems may result in severe economic loss to the operator of the rotary sterilization system.
In order to prevent such loss, a number of approaches have been discussed and proposed for on-line control of sterilization processes. However, all of these approaches concern control of batch sterilization processes performed on a batch of containers {1, . . . , i, . . . , I}batch. In a batch sterilization process, all of the containers generally receive the same time-temperature treatment whether or not a temperature deviation occurs. Thus, when a deviation does occur, a correction to the process can be made which simultaneously effects all of the containers so that a minimum total lethality Fi over [tb, te] will be delivered to the product cold spot of each container i, where tb and te are the begin and end times of the batch sterilization process. An example of such an approach is described in concurrently filed and co-pending U.S. Pat. application Ser. No. 09/187,333, entitled Controller and Method for Administering and Providing On-Line Correction of a Batch Sterilization Process, filed on Nov. 6, 1998, with Weng, Z. as named inventor. This patent application is hereby explicitly incorporated by reference.
In contrast, each container i in a rotary sterilization process will receive a unique time-temperature treatment. Thus, the total lethality Fi over [tf,i, td,i] that is actually delivered to each container is different. This makes it difficult to identify, while on-line and in real time, each container that will have a predicted total lethality delivered to it that is below the target total lethality Ftarg. As a result, the development of a controller that provides on-line handling of a temperature deviation in a rotary sterilization process without stopping the reels of the cooking and cooling shells has been inhibited.
In summary, the present invention comprises a rotary sterilization system, a controller for use in the rotary sterilization system, and a method performed by the controller. The system, controller, and method are used to administer a sterilization process performed on a line of containers and provide on-line handling of a deviation in a scheduled parameter during the process. The containers contain a shelf stable food product that is to be sterilized in the sterilization process. In addition to the controller, the rotary sterilization system includes a rotary sterilizer.
The controller controls the rotary sterilizer in performing the rotary sterilization process according to scheduled parameters. When a deviation in a specific scheduled parameter occurs, the controller identifies those of the containers that will in response have a total lethality predicted to be delivered to them during the rotary sterilization process that is less than a predefined target lethality. This specific scheduled parameter may be a scheduled retort temperature in a temperature zone of the rotary sterilizer through which the line of containers is conveyed. It also may be a scheduled initial product temperature for the containers or a scheduled reel speed for conveying the containers in line through the rotary sterilizer.